A digital radiographic unit and/or a method for taking radiographs in a digital radiographic unit are known, for example, from the article entitled “Flachbilddetektoren in der Röntgendiagnostik” [“Flat image detectors in x-ray diagnostics”] by M. Spahn, V. Heer, R. Freytag, published in the journal Radiologe 43, 2004, pages 340 to 350.
Known in x-ray imaging for the purpose of taking digital radiographs of an object are so-called flat image detectors in which an x-radiation is converted directly or indirectly into an electric charge and subsequently read out electronically by use of so-called active reader matrices and further processed for imaging.
During direct conversion, upon impinging on a direct converter layer, for example made from amorphous selenium, an x-ray quantum of the x-radiation produces a high-energy electron that in turn then generates charge carriers on its way through a direct converter layer. The charge carriers are transported with the aid of an electric field to an electrode divided into pixels, and stored there as a charge; during indirect conversion, upon impinging on a scintillator layer an x-ray quantum of the x-radiation produces a high-energy electron that in turn then generates light on its way through a scintillator layer. The light is converted into an electric charge on a photodiode, arranged below the scintillator layer and divided into pixels, and likewise stored. Subsequently, the corresponding charge pulse, which depends chiefly on the energy of the primary x-ray quantum, is read out via active switching elements assigned to electrode pixels or photodiode pixels in pixel readout units.
A distinction is made between a counting flat image detector and an integrating one. In the case of a counting detector, a charge pulse is judged to be a single x-ray quantum in a pixel readout unit, whereas in the case of an integrating detector integration is carried out over all charge pulses in a pixel readout unit. In order, in the case of a counting detector, to be able to distinguish background noise from a charge pulse originating from an x-ray quantum actually present, there is defined in general a lower threshold value above which a charge pulse is interpreted as a signal of an x-ray quantum.